The broadcasting of multichannel or stereo sound programs for television in the United States is in accordance with the system adopted by the Broadcast Systems Television Committee (BTSC) of the Electronic Industries Association (EIA). This television multichannel sound system provides for the transmission of the sum of the left and right stereo audio information (L+R) in a main audio channel in the spectrum space of the television signal formerly occupied by the monophonic audio signal (mono). This was done so that new stereo signal would be compatible with existing monophonic television receivers. The television multichannel sound system also provides for the transmission of the difference of the left and right stereo audio information (L-R), modulated onto a subcarrier.
The television multichannel system also provides for the option of simultaneously transmitting a second audio program (SAP) which typically may be used to carry a second language version of the program being provided in the main (L+R) audio channel. The audio frequency of the SAP channel is in the range of 50 Hz to 10 KHz, which is relatively restricted as compared to the 50 Hz to 15 KHz for the main channel and the (L-R) subchannel. The restricted bandwidth of the SAP channel has been deemed to be sufficient for the nature of the bulk of the program material expected to be carried on the SAP channel.
The L-R stereo subchannel and the SAP channel both use subcarriers at frequencies above the audio range and due to the nature of the frequency modulation (FM) transmission system used, these channels tend to exhibit relatively high noise levels in comparison with the main audio channel. Companding is therefore utilized for the stereo subchannel and the SAP channel to provide acceptable noise performance. The companding system presently used is referred to as the "dbx Companding System", or simply the "dbx system". It should be noted that dbx is a registered trademark of Carillon Technologies, Inc of Newton, Massachusetts. The dbx Companding System is described in detail in an article by Leslie B. Tyler et al., in the IEEE Transactions on Consumer Electronics, November 1984, Volume CE-30, No. 4, pages 6333-640 and in an article by C. G. Eilers, in the IEEE Transactions on Consumer Electronics, February 1985, Volume CE-31, No. 1, pages 1-7.
The dbx companding system sets out to improve the attainable signal to noise ratio. Fixed and variable preemphasis are applied to the audio signal before transmission. The additional use of variable preemphasis is intended to avoid two problems which may occur when fixed preemphasis is used alone. First, audio signals which contain predominantly high frequencies would be boosted excessively causing overmodulation. Second, low level audio signals lacking sufficient high frequency content would not be sufficiently boosted to mask noise
The transmitter of a dbx system includes a signal compressor in which the L-R and SAP audio signals are passed in cascade through a fixed preemphasis circuit, a variable preemphasis circuit which is responsive to a first control signal, a variable gain stage which is responsive to a second control signal, and a second fixed preemphasis circuit.
The variable preemphasis circuit provides "spectral compression". In spectral compression, the variable preemphasis circuit reduces high frequency gain when the high frequency content of the audio signal is relatively high, because the level of the high frequency portion of the audio signal is sufficient, by itself, to mask noise introduced in the transmission path. Conversely, the variable preemphasis circuit increases high frequency gain when the high frequency content of the audio signal is relatively low, in order to mask noise introduced in the transmission path. The variable gain stage provides "wideband compression". In wideband compression, gain is reduced when the signal level is high, and conversely, gain is increased when the signal level is low.
According to the dbx system, the first and second control signals are generated by first and second control signal generators, each of which comprises a bandpass filter and a "true RMS" detector. A typical "true RMS" detector is known from U.S. Pat. 3,681,618 (Blackmer). The first and second control signal generators provide first and second control signals representative of the energy content of the audio signal over first and second respective frequency ranges.
The inverse of the compression operation, known as "expanding", takes place in the receiver so as to restore the audio signal to its original condition. The received L-R and SAP audio signals are passed in cascade through a fixed deemphasis circuit, a variable deemphasis circuit which is responsive to a first control signal, a variable gain stage which is responsive to a second control signal, and a second fixed deemphasis circuit. The variable deemphasis circuit provides spectral expansion. The variable deemphasis circuit increases high frequency gain when the high frequency content of the received audio signal is relatively high, to restore the level of the high frequency portion of the audio signal. Conversely, the variable deemphasis circuit further decreases high frequency gain when the high frequency content of the audio signal is relatively low, in order to attenuate noise introduced in the transmission path. The variable gain circuit provides wide-band expansion. In wide-band expansion, the gain is increased when the level of the audio signal is relatively high, to restore the original signal levels. Conversely, gain is decreased when the signal level is low, to attenuate noise introduced at the receiver.
Just as in the above-described transmitter, a dbx expander includes two control signal generators, each of which comprises a bandpass filter and a "true RMS" detector, for providing the first and second control signals representative of the energy content of the audio signal over first and second respective frequency ranges.
Unfortunately, the dbx decoder circuitry tends to be somewhat complex and requires a relatively large number of electronic components to implement the functions. This is an undesirable characteristic for a circuit which is to be included within a consumer electronic unit, such as a television receiver, in today's highly competitive market. For this reason, television manufacturers have been searching for a low cost alternative to the costly dbx expansion arrangement. There are television receivers having stereo audio capability, which provide no expansion of the compressed L-R audio signal at all. The model no. RJ 4340, produced by Magnavox Corporation, is such a television receiver. Stereo signals in this receiver are derived by matrixing the sum (L+R) stereo signal from the main audio channel and the unexpanded difference (L-R) stereo signal from the stereo subchannel. Not surprisingly, the stereo image separation performance and noise masking performance of this arrangement has been found to be inadequate over the full range of audio signals received, due to the complete elimination of the dbx decoding circuitry from this receiver. The poor noise masking performance of this receiver causes a perceptible hiss in the reproduced stereo audio sound.